Thanks again for a wonderful product!
One thing I have noticed is that I use the undo move quite a bit to
retry different tactics and strategies, however if I quickly hit undo
repeatedly the state of the board becomes confused and moves will not
be retracted properly. I've even seen situations where a phantom piece
remains on the board (even after I've started a new game) and can only
be fixed by restarting the app (square+power until switch off screen,
then hold square).

The most reliable (but not 100%) method I can give to reproducing is
to make a move and before the animation has completed started pressing
undo. This seems to be the most common case I've noticed as I'm
learning, make and see something I've missed  :)

Hope this helps, would love it I you could make time to fix this
admittedly minor and obscure problem.

Cheers,

Paul